The present invention relates to a photoelectric conversion element employing a new molecular entity (dye) and a solar cell.
In recent years, the utilization of sunlight which is available infinitely and does not generate a harmful substance has been investigated energetically. Inorganic type residential solar cells, such as single crystal silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, indium selenide copper and the like have been practically used as a clean energy source of the utilization of sunlight.
However, these inorganic type solar cells, for example, a silicon type solar cell has defects that extremely high purity is required, naturally, the refining process becomes complicate and is constituted of many processes, and then the manufacturing cost becomes high.
On the other hand, also there are many proposals for solar cells employing organic materials. Examples of organic type solar cells, there are a Schottky type photoelectric conversion element in which a p type organic semiconductor and a metal having a small work function are joined; a hetero junction type photoelectric conversion element in which a p type organic semiconductor and a n type inorganic semiconductor or a p type organic semiconductor and an electronic receptive organic compound are joined; and the like. As the organic semiconductors, employed are synthetic dyes and pigments, such as chlorophyll and perylene; conductive polymers, such as polyacethylene; composite materials of them and the like. These materials are made in a thin film so as to constitute a cell material by a vacuum deposition method, casting method, or dipping method. The organic materials have advantages, such as low cost and easiness in forming a large area. However, they have problems that their conversion efficiency is low to 1% or less, and their durability is not good.
Under such a circumstance, Dr. Gratzel et al in Switzerland proposed a solar cell having good characteristics (refer to Non-patent Document: Nature, 353, 737(1991) by B. O'Regan and M. Gratzel). The proposed cell is a dye-sensitized type solar cell and a wet type solar cell that employs titanium oxide porous thin film spectrally sensitized with a ruthenium complex as a working electrode. The above solar cell has advantages that since a cheap metallic compound semiconductors, such as titanium oxide is not needed to be refined to high purity, the cost is cheap, and since utilizable light extends to a wide visible light region, the cell can convert sunlight including a lot of visible light components effectively into electricity.
However, since the ruthenium complex restricted in resource is used, when this solar cell is put in practical use, the supply of the ruthenium complex is concerned. Further, the ruthenium complex is expensive and has a problem in temporal stability. Therefore, if the ruthenium complex is replaced with a cheap and stable organic dye, these problems can be solved.
There is a disclosure that when a rhodanine skeleton-containing amine structure is employed as such dye, it is possible to obtain an element having high photoelectric conversion efficiency (for example, refer to Patent Document: Japanese Unexamined Patent Publication No. 2005-123033). However, even in the case that such dye is employed, its conversion efficiency is still low as compared with that of the ruthenium complex. Accordingly, sensitizing dye with high photoelectric conversion efficiency is still required.